Method of electrolytically binding a layer of semiconductors together



g- 1968 JAMES E. WEBB ADMINISTRATOR OF THE NATIONAL AERONAuTIcs ANDSPACE ADMINISTRATION METHOD OF ELECTROLYTICALLY BINDING A LAYER 0FSEMICONDUCTORS TOGETHER Filed Nov. 10, 1964 lgaL S/DNEV G. ELL/5INVENTOR.

ATTORNEYS United States Patent 3,396,057 METHOD OF ELECTROLYTICALLYBINDING A LAYER 01F SEMICONDUCTORS TOGETHER James E. Webb, Administratorof the National Aeronautics and Space Administration, with respect to aninvention of Sidney G. Ellis, Princeton, NJ.

Filed Nov. 10, 1964, Ser. No. 410,330 7 Claims. (Cl. 136-89) ABSTRACT OFTHE DISCLOSURE This invention teaches a method of binding a layer ofsemiconductor particles together. Electro-deposition is used to form aninsulating film that holds the particles together. The particles can beused in making solar cells, photoelectric cells and other semiconductorproducts. Essentially, the particles are deposited on the surface ofmercury. The particles float on the mercury. They are compacted and thencovered with an electrolyte. One electrode is inserted in theelectrolyte while the mercury serves as the other electrode. After thelayer is formed, the level of the mercury is lowered so that the nowformed layer will adhere to a base member previously submerged in themercury.

Origin of the invention This invention described herein was made in theperformance of work under a NASA contract and is subject to theprovisions of Section 305 of the National Aeronautics and Space Act of1958, Public Law 85-568 (72 Stat. 435, 42 USC 2457).

Field of the invention This invention relates to a method of producing asemiconducting product having a single monoparticle layer ofsemiconductor material and more particularly to a method of providing asingle monoparticle layer on a base wherein the particles are boundtogether by an insulating film so that portions of the particles areexposed to receive subsequent coatings, formations, contacts, etc.

Description of the prior art In the manufacture of solar cells, seleniumcells, photoelectric cells and other semiconductive products, one of theconventional practices has been to coat or apply an amorphoussemiconductor material to a metallic base by pressing it cold into thebase. For this purpose, the semiconductor material, such as selenium,for example, is made into powder form and is arranged on the basefollowed by fixing the powder thereto through the application of highmechanical pressure. Subsequently, the fixed powdered base istransformed from its amorphous condition into a crystalline condition.After subjecting the crystalline powdered base to high temperatureprocessing, the powder layer is completely transformed into the metalliccondition.

Another practice employed by the prior art involves the floating ofsemiconductor fluid repellent particles on the surface of a liquidfollowed by compressing or compacting the particles together to form acontinuous layer and subsequently raising a previously submerged basemember through the liquid to deposit the particle layer on the basemember.

Difficulties have been encountered in practicing these conventionalmethods because of the time and heating requirements needed for the highmechanical pressure method and the necessity of making the particlesliquid repellent in the latter method.

These difliculties are obviated through the employment of the method ofthe present invention wherein a semi- Cir 3,396,057 Patented Aug. 6,1968 "ice Summary of the invention In general, the method of the presentinvention provides for the floating of a layer of semiconductingmaterial in powder form on the surface of a fluid having a high densitysuch as metallic liquid represented by mercury, for example. Thesemiconducting powder is randomly sprinkled on the surface of themercury and is supported by the mercury. To effect compaction of theparticles, the area over which the particles have been sprinkled iscaused to be reduced so that a single layer of particles is arranged.The mercury and the particles are covered with a solution compatiblewith mercury to be used for electrodeposition. A material iselectro-deposited to form an insulating film that will hold theparticles together about their respective equators. During theelectro-deposition portion of the present method, the mercury iselectrically conductive and serves as the positive anode. The particlefilm is deposited on a base previously submerged in the mercury.Therefore, a semiconductor product is provided which includes aninsulating matrix deposited electrolytically around the midsections orequatorial planes of the semiconductor particles leaving their polarregions free and exposed for junction, barrier or contact formation.

An object of the present invention is to provide a method of producing apowder coated product whereby the powder forms a uniform singlemonoparticle layer wherein the particles are insulated from each other.

Another object of the present invention is to provide a method forproducing a monoparticle powdered film whereby the particles are, inessence, bound together at their respective centers or midsectionsleaving the opposite polar regions or ends of the particles exposed andfree for other types of contact formations or coatings.

Still a further object is to provide a method for forming a powderedcoated product whereby an oxide layer serves to bind the powder togetherinto a monoparticle film as well as to serve as a base for supportingthe film.

Another object resides in the feature that the aforementioned fluidserves not only as a support for the powdered particles but the fluid isused as an electrode during the oxidized process.

Brief description of the drawing The features of the present inventionwhich are believed to be novel are set forth with particularity in theappended claims. The present invention, both as to its organization andmanner of operation, together with further objects and advantagesthereof, may best be understood by reference to the followingdescription, taken in connection with the accompanying drawings, inwhich:

FIGURE 1 is a cross-sectional view of a typical product formed by themethod of the present invention;

FIGURE 2 is a cross-sectional view of a container for holding mercuryhaving semiconductor material particles supported on its surface asemployed in the manufacture of the product of FIGURE 1;

FIGURE 3 is a view similar to that of FIGURE 2 showing compaction of theparticles into a layer one particle thick effected through the reductionof mercury surface area by raising its level;

FIGURE 4 is a view similar to FIGURE 3 illustrating the addition of asuitable solution into the container over the compressed particle layercompatible with mercury for oxidation to form an insulating film holdingthe particles together; and

FIGURE is a view similar to FIGURE 4 illustrating the depositing of theelectro-deposited monoparticle layer onto a supporting screen.

Description of the preferred embodiment In the practice of thisinvention, the powder semiconductor material to be coated onto a basemember is not required to be naturally liquid repellent or renderedrepellent to the liquid carrying media on which the materials arefloated in order to prevent the powder materials from sinking in themedia. While various liquids may be chosen for supporting thesemiconducting material, it is preferable to use a liquid media which isof greater specific gravity than the semiconductor materials so that thematerials will float on the surface of the liquid media. A liquid metalsuch as mercury may be used as a supporting media for the semiconductormaterials. Because of the density of mercury, particles of thesemiconductor material will not be readily absorbed into the liquid orthe liquid absorbed into the particles. Therefore, a feature of theinvention resides in employing a fluid media which does not firstrequire treating the particles to prevent them from sinking into thefluid. While a specific metallic fluid medium is given herein forsupporting the semiconductor material particles, other well-known highdensity media, which are intended to be covered herein, will occur tothose skilled in the art.

The semiconductor material particles, which need not be eitherinherently water repellent or artificially liquid repellent, may beplaced on the surface of the mercury in any convenient way. The powdermay be deposited or scattered so that the particles readily spread overthe surface of the liquid. The powder may be applied through a screen orcloth, through an air-blown spray, sprinkling or dusting, or otherwell-known methods. After depositing the powder on the surface of themercury, the powdered particles are relatively widely dispersed. Inorder, therefore, to obtain a continuous coating which is one particledeep or thick, it is necessary to compress the deposited film orcoating, or reduce its areas, so that the particles are rearrangedrelatively close together.

In order to maintain and fix the rearranged particles in close proximityto each. other, an insulating film is provided. The film is generated byan oxidation process through electrolysis by introducing a chemicalsolution compatible with the mercury for causing oxidation to occur.When the liquid is added, it wets the sides and top of the particles butnot the bottoms which remain wetted by the supporting mercury. In somecases, depending on the composition of the particles used, it may benecessary to pretreat part of the surface of the particles so thesurface will be Wetted by the mercury to prevent the solution fromgetting between the particles and the mercury. A liquid solution thatmay be used is represented by a soluble tungstate which is employed tocover the compressed particle arrangement on the surface of the mercury.Employing the mercury as a positive anode and applying a negativevoltage to the tungstate, oxidation occurs which forms an insulatingfilm at the junction of the tungstate and mercury. This film adheres tothe midsection of the particles so that the opposite polar regionsextending on the tungstate solution and the mercury respectively remainfree and unencumbered by the film.

A clean liquid surface is necessary to insure uniform and constantbehavior of the powder on the mercury surface. The powder is depositedon the mercury surface by sifting through a screen, sprinkling, dusting,spraying or by any other convenient method. Just enough powder isdeposited to give a dispersed particle arrangement, such as that shownin FIGURE 2. In order to have a continuous, uniform layer or coating ofparticles one particle thick, it is necessary to compress or concentratethe dispersed particles into a compaction aggregation, such as is shownin FIGURE 3. This may be readily accomplished by the raising of thelevel of the mercury in the container so that the inwardly disposedsidewalls of the container force the particles into a concentratedarrangement. By raising the level of the mercury, the concentration ofpowder particles per unit area of mercury surface is increased. If theparticle concentration is so increased until incipient cracks orwrinkles begin to form, a continuous coating one particle deep, as shownin FIGURE 3, is attained.

Inasmuch as the particles are of greater density than the tungstatesolution, the particles are not absorbed by the solution covering. Theelectrolytic process is initiated by applying a negative potential tothe tungstate solution while a positive potential is applied to themercury. Oxidation occurs which results in a thin insulating film whichbinds the particles together into a monoparticle layer. To achieve thefullest effect of the electrolytic process, it may be desirable toprevent the tungstate solution from settling between the particles andthe mercury.

A thoroughly cleaned article or base member such as a screen orconducting base member whose upper surface is to be coated, ismaintained submerged within the fluid medium beneath the formedinsulating film particle layer. After the oxidizing step, the mercurymay be withdrawn from the container to lower the formed insulated filmparticle layer onto the base member. Undersirable extremities of theparticle layer will break off from the layer being deposited on the basemember as the mercury level continues to be lowered inasmuch as the basemember is stationary as it receives the film layer.

For a specific example of the operation of the present method, referenceis made to the drawing wherein FIG- URE 1 illustrates a typicalsemi-conductor product manufactured by the method of the presentinvention. A thin monoparticle layer 10 is shown bound together by anoxide film 11 which is mounted on a base member 12, such as a screen offine mesh. It is to be noted that particles are joined by the film abouttheir mid-sections and that the top and bottom of the particles are freeof the film so that the :bottoms of the particles come into immediatecontact and union with the base member. The tops of the particles arecovered with a suitable conducting layer 13. A contact .14 may be fixedto the layer 13 by suitable methods such as evaporation or welding. Tocomplete an electrical product, a suitable conducting barrier 15 may besecured to the side of the base member opposite to its side engaged bythe monoparticle layer if the layer 12 does not itself have theappropriate electrical mechanical properties. Other uses and electricalproducts employing a single monoparticle layer will occur to thoseskilled :in the art. FIGURE -2 shows a container 16 having an annularinwardly inclined sidewall 17 and a fluid entrance port 18 located inthe bottom thereof. Mercury 19 is introduced into the container via port18 which offers a surface 20 for supporting powdered particles such asrepresented by particle 21. A base member 22 is shown submerged in themercury below the particle scattered surface. The member may besupported by any convenient means; however, a rod 23 having a handle 24is employed in the illustration for supporting the member as well as foreffecting the removal of the member after the particle layer has beendeposited thereon. The vertical part of rod 23 should have an insulatingcoating on it. It is to be noted in this view that the scatteredparticles are randomly disposed and loosely arranged.

FIGURE 3 illustrates the step of compacting or compressing the looseparticle arrangement into a single monoparticle layer by effectivelyreducing the surface area over which the particles are distributed. Thisarea reduction is achieved by introducing more mercury into thecontainer via port 18 to raise the surface level of the mercury. As thesurface level rises, the inwardly inclined sidewall .17 of the containerurges the particles together into a relatively compact singlemonoparticle layer 25.

Following the compaction of the particles, FIGURE 4 illustrates the stepof depositing an insulating film by introducing a quantity of tungstatesolution 26 into the container to cover the monoparticle layer 25. Theweight of the particles prevents the solution from flowing between theparticles and the mercury. In some instances, it may be desirable tocoat the bottom of the particles with an agent which may be wetted bythe mercury to insure against the flow of the solution. The solution maybe introduced by any suitable method, but it is suggested that a smallpipette be employed for this purpose so that the solution, as it isadded, will not disturb the compact particle arrangement.

An electrolytic film 11 is provided by dipping an electrode 27 into thetungstate solution and applying a positive potential to the mercury,using the mercury as an anode. The film interconnects the particles attheir equators leaving their opposite polar regions free and exposed forfurther formation or processing. It is to be noted that FIGURE 4 shows asingle compact monoparticle layer bound together by an oxide film.

With reference to FIGURE 5, the single monoparticle layer is shown beingdeposited onto the base member 22 by removing the mercury from thecontainer via port 18 which lowers the surface level of the mercury. Asubstantial quantity of mercury is removed so that surface level dropsbelow the previously submerged base member. Preferably, the base memberis arranged angul-arly to the surface level so that the monoparticlelayer is progressively deposited thereon as the mercury is removed. Itis to be noted that the monoparticle layer extends substantially acrossthe entire length of the mercury surface. However, upon the progressivedeposition of the layer onto the mesh 22, undesired extremities 28 ofthe layer break away from the main central body 29 portion of the layerwhich is supported on the mesh.

In the manufacture of other semiconductor products, such as solar cellsfor example, it is desirable to make ohmic contact to the bottoms of theparticles by film 12 and then contact the tops with a transparentconducting layer 13 which would form a photovoltaic barrier with theparticles. In this manner, layer 15 is unnecessary.

In the manner described above, the method of the present inventionprovides a powder coated product as illustrated in FIGURE 1. Morespecifically, the method provides a container with an inwardly inclinedsidewall which is partially filled with mercury. The mercury is fedthrough the ported bottom portion of the container so as to raise orlower the top surface level of the mercury without disturbance. Asemiconducting powder is sprinkled on the mercury surface followed byraising the level of the mercury so that the inclined sidewalls compactthe particles. Next, the mercury and compacted particles are coveredwith a tungstate solution to be employed for oxidation. The mercury is,then, covered with an insulating film that will hold the particlestogether at their midsections. The mercury, being conductive, serves asthe positive anode during oxidation. Finally, the mercury level islowered to deposit the monoparticle film layer onto a mesh or platepreviously submerged in the mercury.

While there have been shown and described and pointed out thefundamental novel features of the invention as applied to a preferredembodiment, it will be understood that various omissions andsubstitutions and changes in the form and details of the deviceillustrated and in its operation may be made by those skilled in theart, without departing from the spirit of the invention; therefore, itis intended that the invention be limited only as indicated by the scopeof the following claims.

I claim:

1. A method of manufacturing a powdered semiconductor material productcomprising the steps of:

floating powdered semiconducting material on the surface of a liquidhaving a greater density than that of the material; electrolyticallygenerating an insulating film at the surface of the liquid in suchmanner to interconnect said floating powdered semiconducting materialtogether into a single monoparticle layer; and adhering said generatedfilm layer onto a base member. 2. A method of forming a singlemonoparticle of semiconductor material comprising the steps of:

floating particles of semiconducting materials in a confined arrangementon the surface of a liquid having a greater density than that of thematerial;

electrolytically generating an insulating film at the surface of theliquid in such manner to interconnect said floating semiconductingmaterial particles together at their mid-sections into a singlemonoparticle layer whereby the opposite end regions of the particles arefree and exposed; and

adhering said generated film layer onto a base member.

3. A method of producing a monoparticle layer comprising the steps of:

supporting a plurality of particles in a confined random arrangement ona fluid which has a greater density than the particles;

reducing the confined area to compress said supported particles;

electrolytically producing an insulating film at the surface of theliquid joining said compressed particles into a layer in such mannerthat the film joins the particles about their equators leaving theiropposite polar regions free and exposed; and

adhering said oxidized particle layer onto a base member.

4. A method of producing a single monoparticle layer of semiconductormaterial comprising the steps of:

floating a plurality of semiconductor material particles in a confinedrandom arrangement on an electrically conductive liquid which has agreater density than that of the material;

reducing the confined area to compress said floating particles;

electrolytically producing an insulating film at the surface of saidliquid, that joins the particles about their equators leaving theiropposite polar regions exposed; and

adhering said particle layer onto a base member so that one surface ofsaid layer remains free and exposed. 5. A method of producing a singlemonoparticle layer of semiconductor material comprising the steps of:

floating a plurality of semiconductor material particles susceptible toliquid penetration in a confined random arrangement on the surface of aliquid which has a greater density than that of the material;

reducing the area of the particle arrangement to compress said floatingparticles;

electrolytically producing an insulating film at the surface to theliquid to join said compressed particles into a single layer in suchmanner that the film joins the particles about their equators leavingtheir opposite polar regions free and exposed; and

adhering said particle film layer onto a base member previouslysubmerged in the liquid by lowering the liquid level.

6. A method of manufacturing a monoparticle layer wherein each particleis electrically insulated from adjacent particles comprising the stepsof:

supporting an arrangement of particles on the surfaces of a liquid metalin a confine-d area;

reducing the area of said confined particle arrangement to coimpress theparticles together;

introducing a solution over said compressed particle arrangementcompatible with the liquid metal for effecting oxidation;

passing an electrical current through said solution and metal to form aninsulating film that joins said particles into a single monoparticlelayer; and

7 8 adhering "said single monoparticle layer onto a base merged in themercury by removing mercury to lower member. its surface level; and 7. Amethod of manufacturing a monoparticle layer removing said particlefilm-bearing mesh from the wherein each particle is electricallyinsulated from admercury. jacent particles comprising the steps of: 5References Cited supporting an arrangement of particles on the surfaceUN T STATES PATENTS P a ma; 2,359,920 10/1944 Kelleric 204416 reducingthe area of said confined particle arrange- 2633 426 3/1953 Kouer' mentto compress the particles together by raising 3:039:896 6/1962Cakenberghe e level-mercury; 10 3,070,866 1/1963 Kastenbein 136-298 Xcove i g said compressed particle arrangement with 21 3,226,263 12/1965Oswin tungstate solution; passing an electrical current through saidmercury and FOREIGN PATENTS tungstate solution to form an insulatingfilm that 631,709 1/1962 yhldth rt'l t t o s e pa to es toge her a the1rIIlld sections, ALLEN B. CURTIS, Primal? Examineradhering said particlefilm on a mesh previously sub-

